In recent years, the densification of imaging devices is advancing, and there has been a significant increase in the level of high-definition and miniaturization of digital video cameras and digital still cameras. When the element-density of an imaging device increases, the surface area of light-collecting elements decreases, and a long exposure time becomes necessary to ensure Signal-to-Noise (SN) ratio. Since image blur occurs when the camera position moves during exposure, an image-capturing person takes measures such as steadying the camera on a tripod or steadying his arms in the case of a hand-held camera, in order to keep the camera from moving.
In order to reduce the burden of image blur prevention on the image-capturing person, models in which the camera is equipped with an image blur correction function have been brought to practical use. The basic idea is to detect the motion of the camera and obtain image-captured pictures so as to cancel out such motion. The specific method for canceling out camera motion can be broadly classified into two, namely, “electronic” and “optical”. The electronic-style cancels out image blur using a method of clipping out a part of an image-captured picture and moving the clip-out position in the opposite direction of the camera motion.
FIG. 34 is an explanatory diagram describing an example of the electronic image blur correction function.
For example, as shown in FIG. 34(a), a camera sets a clip-out frame 11 which is smaller than an image-captured picture 10. The camera outputs the picture inside the clip-out frame 11 as an output picture 12, and displays the output picture on a display or stores it in a recording medium. An image-captured picture 13 in FIG. 34(b) is a picture that is one frame ahead of the image-captured picture 10. Camera motion 14 occurs between the image-capturing in FIG. 34(a) and the image-capturing in FIG. 34(b), and even when a subject 15 is still, the in-picture position of the subject 15 moves, as in output picture 16, when there is no image blur correction function. The difference between the output picture 12 and the output picture 16 is the “image blur”. When the camera does not move and the subject also does not move, there should be no change in the position of the subject in the output picture.
Consequently, a camera having an image blur correction function solves the image blur and obtains an output picture 18 which is the same as the output picture 12 by moving the clip-out frame 11 according to a camera motion correction vector 17 which is in the opposite direction and of the same size as the camera motion 14, as shown in FIG. 34(c).
On the other hand, a camera having an optical image blur correction function mechanically moves the lens system and/or imaging device so as to cancel out the camera motion. Therefore, image blur correction is performed on the picture formed in the imaging device itself.
As described above, with the electronic image blur correction, it is necessary to accurately detect the motion of the camera since the clip-out frame 11 is to be moved according to the motion of the camera as described using FIG. 34. In addition, such motion of the camera is detected based on the motion of the image-captured picture. When the subject is still, it is easy to accurately detect camera motion. However, in actuality, there are cases of image-capturing a moving subject and, in particular, with a video camera it is very rare that the subject is still since the primary purpose is to record video which temporally pursues the motion of the subject.
Consequently, for example, Patent Reference 1 discloses an electronic image blur correction technique which allows image blur correction even when a moving subject is image-captured.
FIG. 35 is a configuration diagram showing the configuration of the motion vector detecting device included in the image-capturing apparatus of the aforementioned Patent Reference 1. The motion vector detecting device includes a motion vector detecting unit 31, a motion vector judging unit 32, a divergence degree calculating unit 33, LPF 34, a transforming unit 35, and a control unit 36. The motion vector detecting unit 31 detects the motion vector for each block of an image-captured picture.
FIG. 36 is a diagram showing the motion vector for each block. As shown in FIG. 36(a), the motion vector detecting unit 31 divides an image-captured picture 20 into plural blocks and detects the motion vector for each block. When the subject is still, the motion vectors of all the blocks are identical, which indicates camera motion. On the other hand, when there is a moving subject, the motion vectors of all the blocks are not identical. Therefore, the motion vector detecting unit 31 detects motion vectors having different orientations, as shown in FIG. 36(b). Consequently, the divergence degree calculating unit 33 calculates a “divergence degree” which indicates the degree of divergence of the motion vectors. Then, the control unit 36 calculates the motion vector for the whole of the image-captured picture 20 (camera motion) according to the divergence degree. Specifically, the transforming unit 35 sets a relationship between the divergence degree and a motion vector coefficient which controls the size of the motion vector for the whole image-captured picture and, as the divergence degree increases, the transforming unit 35 suppresses the size of the motion vector for that divergence degree.
FIG. 37 is a diagram showing the relationship between the motion vector coefficient and the divergence degree. The transforming unit 35 suppresses the effects of a motion vector of a large divergence degree on the motion vector for the whole image-captured picture by decreasing the motion vector coefficient for a motion vector of a higher divergence degree. Based on the result of such processing by the transforming unit 35, the control unit 36 controls the size of the motion vector for the whole image-captured picture.    Patent Reference 1: Japanese Patent No. 2506469.